Add hghave utility and run-tests.py support. hghave detects system features like symlinks availability at run-time. Tests can be skipped by starting them with: "$TESTDIR/hghave" symlink || exit 80 The 80 exit status triggers hghave output handling by run-tests.py. Also, tests output can be locally patched on the fly to match reference output.

run-tests: extract correct status when script terminates with exit

Merge with -stable